Cholinesterase (ChE) activity is a recognised biomarker of organophosphorus (OP) insecticide exposure. It is frequently applied to earthworms in the field of agriculturally linked ecotoxicology, because of the use of OP insecticides on crops, and the exposure of earthworms due to their inhabiting the soil. The ubiquitously occurring metal lead (Pb) has previously been shown to both influence and not influence ChE activity in earthworms, therefore it was of interest to investigate the influence of lead on chlorpyrifos-induced ChE inhibition in the earthworm Eisenia fetida. Four treatment groups were exposed to chlorpyrifos (10 mg/kg), and three of the groups were also exposed to lead (20, 500 and 1000 mg/kg). Worms were sampled for analyses after 6 hours and after 1 week of exposure. In addition to the ChE activity, the amount of DNA strand breaks in coelomocytes was measured, thereby allowing a comparison of these two biomarkers. After 6 hours, there were no significant differences between the ChE activity between any of the treatment groups and the control. The DNA strand breaks could not be assessed at this point due to lack of enough cell samples of good quality. After 1 week, the ChE activity in the worms exposed to chlorpyrifos alone and combined with 20 and 1000 mg/kg lead was significantly lower than the control (P < 0.05), corresponding to a 35-38 % inhibition. No effect of lead on the chlorpyrifos-induced ChE activity could be stated. DNA strand breaks as expressed by tail extent moments (TEMs) increased significantly compared to the negative control in the worms exposed to chlorpyrifos and the two highest concentrations of lead (P < 0.01), whereas no significant differences in Olive tail moments (OTMs) were detected. The syringe extraction method was used when sampling coelomocytes from the worms, and based on the results, it is not considered a preferable method when used as described here.
OP insecticides are known to cause an irreversible inhibition of ChE activity, which means that new enzyme must be synthesised for the activity to recover. Recovery of ChE activity after exposure to several OP insecticides has in many animals, including earthworms, been proved a lengthy process, however little is known of the recovery of earthworms following chlorpyrifos exposure. To see whether the slow ChE recovery applies to chlorpyrifos as well, E. fetida specimens were exposed to chlorpyrifos (240 mg/kg) for 48 hours, and subsequently transferred to clean soil. The activity of both types of ChE in E. fetida, E1 and E2, was measured at intervals over a 12 week period, and the appearance and behaviour of the worms were observed. Directly after exposure, the total ChE and E2 activity was inhibited by 63 % and 52 %, respectively, and there was no rise in the activity of the exposed worms compared to the controls during the recovery period. However, after 2-3 weeks, the exposed earthworms could not be distinguished from the control worms by appearance or behaviour. Aspects of applying ChE activity as a biomarker are discussed.